Such an apparatus is known from Dutch patent NL187945. An example of such an apparatus is represented in FIG. 1. FIG. 1 shows in particular a reference amplifier known per se which is provided with the several channels with signal inputs 1 for receiving input signals E1, E2, E3, . . . , En. The input signals come in particular from sensor electrodes (not represented in FIG. 1). The input signals can each comprise a specific signal component as well as a signal component common to all input signals. Each channel is provided with an impedance transforming input amplifier 3. The apparatus is configured for supplying to the non-inverting input (in the drawing each time provided with the “+” mark) of each amplifier 3 a respective input signal and to the inverting input (in the drawing each time provided with the “−” mark) an analogue reference signal Vref which is equal for all channels, for providing an associated signal via an output of the input amplifier 3. The known apparatus is further provided with an analogue averager 1006 for forming the reference signal Vref. Further, a second amplifier stage 1049 is provided (see FIG. 1) for removing the common signal component from the signals. Furthermore, one or more differential amplifiers 1050 are provided, in particular for each time further amplifying two associated signals coming from the second stage 1049 in the case of a bipolar input signal measurement. For a further discussion of various components of such an apparatus and particular advantages of the known apparatus, reference is made to the content of NL187945.
It is furthermore known, with the known system, to further provide each channel with a band pass filter 1007 of which an input is coupled to the output of the second amplifier stage 1049. Each band pass filter 1007, which is typically used in the known apparatus, comprises a low pass filter part and a high pass filter part for allowing passage of a particular frequency band BP, which band B, in particular, does not contain DC components.
It has appeared that with particular measurements, for instance electrophysiological measurements, the input signals E1, E2, E3, . . . , En, can each contain various components. A first component in each signal comprises, in particular, the electrophysiological information. A second component comprises, in particular, a mains interference signal. A third component comprises a sensor offset signal (in particular an electrode offset signal).
The electrophysiological information mentioned is in particular the important measurement part of the input signal and can have an amplitude in the range of, for instance, some microvolts to some millivolts.
Mains interference, in particular resulting from 50 Hz or 60 HZ mains AC voltage, is also called a “common mode disturbance”. The amplitude of this interference can vary strongly, between, for instance, 1 and 100 mV. As a rule, the reference amplifier system represented in FIG. 1 can quite well remove this interference component from the signals, as the mains interference component will form an important part of the reference signal.
A sensor-offset signal, in particular an electrode offset (the third component in an input signal) can have varying amplitudes, and is generally not stable. It can comprise a DC signal with an amplitude between 0 V and some hundreds of mVs, and can vary with a very low frequency. During use of the apparatus represented in FIG. 1, an average of these offsets also forms part of the reference signal mentioned. Here, the variance in the different offsets of the different inputs signals E1, E2, E3, . . . , En is amplified by the apparatus as if this variance were part of the electrophysiological information signal component. The mutual variance in the offsets can be relatively large, which means that the gain in the first amplifier stage (comprising the input amplifiers 3) of the apparatus shown in FIG. 1 is to be limited. If a said offset variance is for instance 150 mV, the gain is for instance only, at most, a gain 20.
The high pass filter part of the apparatus is intended for removing the offset from the signal, both a non-amplified average offset and an amplified offset variance. As a result, it is possible to have the second amplifier stage amplify the remaining signal once again, for instance 100 times. A physiological signal can have an amplitude of, for instance, 0.1 mV, while the offset variance can already be 150 mV. The offset itself can be as much as 500 mV. To this end, each high pass filter may have a relatively low cut-off frequency of 0.01 Hz, so that it is substantially avoided that DC signals can reach the differential amplifier stage.
A drawback of the known apparatus is that it feeds back an average sensor offset per channel, wherein variances in offset per channel determine how high the amplification can be chosen. When using different sensor-electrode materials, and/or if electrodes make poor contact, the apparatus can attain a saturation mode.
For instance, an artefact in one input signal, for instance a movement artefact or an artefact resulting from a stimulus, can form a relatively large offset in this signal, for instance an offset of a few hundreds mV. This means that the first and second amplifier stages (3, 1049) are saturated. The input amplifier stage 3 and second stage 1049 can leave this saturated mode relatively rapidly, generally within some milliseconds. It has however appeared that the amplifier stage 1050 (which can amplify for instance 100×) can remain out of range for several minutes so that a measurement, comprising the signal processing to be carried out by the apparatus, remains blocked for an undesirably long period of time.